FAP-221C - offline/disconnected

On a Fortigate 60E with v6.2.3 - build1066 I have 3 FAP-221C FortiAP's successfully "connected". Thus 3 AP's have a fortiprofile connected to them and are working and having clients etc.

Now when I want to connect a 4th FAP-221C I have issues. Whatever I tried I can't get this AP "connected". The AP has the latest FW 6.0 Build 0044. I upgraded it via CLI (via 192.168.1.2 etc). If connected that way I can login via SSH and via browser. I have done "factoryreset" and cfg -x the AP and it boots up via POE and via 12V (if I go to it via 192.168.1.2) etc.

But when connected to the same interface as the other 3 AP's, having an IP address, can ping from Fortigate, etc. It just does not get "connected"...

What can be wrong? the only thing different is that the 3 working APs are model "FAP-221C-E" and the problematic one is "FAP-221C-I"...

Please help me out :)

EDIT: Now after 1 hour... I see to great astonishment that the AP is sending out a "test123" SSID... just out of the blue despite the FAP has been factory defaulted multiple times via CLI and via the "paperclip... I can successfully connect to that test123 SSID and access my local network instantly without any password... and in the Fortigate everything is the same Ehm... What the heck?